pep508: remove hidden state from `MarkerTree`
I split this change into its own commit because I'm hoping it
crystalizes what it means when we say "a `MarkerTree` has hidden state."
That is, it isn't so much that there is some explicit member of a
`MarkerTree` that is omitted, but rather, the lower and upper version
bounds on `python_full_version` are are rewritten as "unbounded" when
traversing the ADD for display.
We will actually retain this functionality, but rejigger it so that it's
explicit when we do this. In particular, this simplification has been
problematic for us because it fundamentally changes the truth tables of
a marker expression *unless* you are extremely careful to interpret it
only under the original context in which it was simplified. This is
quite difficult to do generally, and in prior work in #6268, we
completed a refactor where we worked around this type of simplification
and moved it to the edges of uv.
In subsequent commits, we'll re-implement this form of simplification as
a more explicit step.
